Power supplies have an efficiency rating: the higher the efficiency, the less power wasted, and/or the less heat generated. For example, a power supply whose efficiency rating is 80 percent provides 80 percent of its rated wattage to the system it is powering and loses 20% in the form of heat.
Available power supplies include stand-alone (“external”) devices, as well as those built into larger devices along with the larger devices' respective loads. Examples of the latter include power supplies embedded in desktop computers and consumer electronic devices. Currently, an external power supply is used in many PC systems. For Notebook PC systems this has been the common design practice for some time. As DT (desktop) systems become smaller and smaller, use of external power supply has become a common design practice for DT computers as well.
Conventionally, the limit on current to be drawn by the computer (say) from the external power supply is set to maximal value based on worst case scenario with maximal ambient temperature. The maximal ambient temperature may, conventionally, be determined based on expected usage.
Once the maximal ambient temperature has been determined, the maximal current (limit) is derived, e.g. using a suitable formula which expresses the relation of ambient temperature and maximal power consumption and is dependent on the power supply implementation. Since power supplies are not 100% efficient, the power lost inside the power supply generates heat. For a specific loss, a specific gradient between power supply temperature and external temperature is needed. So, if the ambient temperature is low, the power supply temperature will be lower, in which case the internal electronic components of the power supply are colder. Often, electronic components are more efficient in lower temperatures and can therefore deliver more current compared to hot components. For example an on-resistance vs. junction temperature graph for a 2N7002W FET transistor (say) shows an increase in drain source resistance over temperature. Power loss over the FET is proportional to resistance. When the temperature is higher, power loss over the FET is higher. So, at low temperature more current can be transferred by the FET with the same power loss.
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